This afternoon the European Union bill begins its committee stage – where the bill is subjected to detailed, line-by-line scrutiny. The house approved the principle behind the bill at second reading before Christmas. Unusually, the EU bill is being considered by a committee of the whole House of Commons, a procedure used for controversial bills.

What does the EU bill do?

The bill fulfils two Conservative manifesto promises: to require a referendum before any further transfer of power to the EU, and to introduce national sovereignty legislation to make it clear that authority over our laws stays in this country. The coalition agreement committed the government to introduce a "referendum lock" and to examine the case for a sovereignty bill.

The main provisions of the bill are:

• To require a referendum before the government could agree to change the current EU treaties, or to certain EU decisions, so as to transfer power to the EU.

• To require an act of parliament before the UK could agree to a number of other specified decisions provided for in the existing treaties.

•Clause 18 makes clear that EU law has effect in the law of the UK only through an act of parliament.

Does the bill repatriate powers from the EU to the UK?

No. It does not affect any existing EU competence, whether transferred in the Lisbon treaty or earlier.

When will a referendum be required?

A referendum will be needed whenever the EU treaties are changed to transfer power to the EU, though some less far-reaching changes can be approved without a referendum if not "significant". No referendum will be needed if a new member state (such as Turkey) joins the EU.

How do we know whether or not a treaty change transfers power to the EU?

A minister will decide whether power is being transferred. That decision can be challenged in court, which has led Yvette Cooper, the shadow foreign secretary, to say the bill creates "a lawyers' paradise".

Couldn't a future government just repeal the "referendum lock" anyway?

Yes. The bill does not include any entrenchment mechanism (for instance requiring a Commons vote of 75% to repeal the referendum lock), so a future government could repeal it in order to avoid a referendum. This has led the Conservative MP Douglas Carswell to call the bill "a piece of legislative PR".

How does the bill guarantee national sovereignty?

Clause 18 confirms the existing position, that EU law only has effect in the UK through an act of parliament. The government argues that this will ensure the courts reject any argument that EU law has authority in Britain in its own right.

Does clause 18 affect the supremacy or primacy of EU law?

No. Clause 18 does not alter the existing relationship between EU law and UK law or affect the primacy of EU law.

So what's the problem with clause 18?

Because clause 18 merely restates the existing position rather than cutting down the power of the EU, it does not go far enough for Eurosceptics. The EU scrutiny committee has said clause 18 is not really a sovereignty clause but that the government portrays it as such "for political reasons". Professor Adam Tomkins of Glasgow University has said the clause could even be dangerous, since it could have unforeseen effects on parliamentary sovereignty.